1. Field of the Invention
This invention generally relates to semiconductor devices and to methods of fabricating semiconductor devices. More specifically, the invention relates to the formation of a high performance varactor on silicon in the manufacture of integrated circuit devices.
2. Background Art
Varactors are voltage variable capacitors. These devices are essential for the design of key radio frequency (RF) CMOS and BiCMOS circuits, and are specifically used as tuning elements in voltage controlled oscillators (VCCs), phase shifters, phase-locked loop (PLL) circuits, and frequency multipliers.
In general, varactor designs must maximize a number of properties. One is “tunability,” which is the ratio between the highest and lowest capacitive values (Cmax/Cmin) over the range of applied voltages for the circuit. Another is “linearity.” There are two definitions of ‘linearity’: 1/sqrt(C) and d(LnC)/dV, where C is the voltage-dependent varactor capacitance. In the first case it is desired that 1/sqrt(C) be a straight line, and in the second, that d(LnC)/dV be a constant, both as V varies. Yet another property is “Q,” or quality factor, which a function of the series resistance of the diode and the capacitive value of the varactor at the higher frequency ranges of the circuit.
In order to enhance the capacitive swing of a varactor, it also known to vary the dopant concentration of one or both of the diffused electrodes of the diode such that the diffusion has a retrograde dopant profile (that is, the dopant concentration is higher at the lower portion of the diffusion region than it is in the top). These so-called “hyperabrupt” junctions greatly increase the change in varactor capacitance for a given voltage swing.
In practice, it has proven to be difficult to simultaneously enhance tunability, linearity, and Q of a varactor when integrated into a CMOS or BiCMOS process. For example, considering the PFET source/drain junction and well as a varactor device, additional n-well implants will decrease the well resistance and increase varactor Q, but will decrease varactor tuning range by making the source/drain p-n junction depletion regions smaller.
Accordingly, a need has developed in the art for a varactor design that optimizes the tradeoffs between all of these properties, particularly when integrated into a process for forming other integrated circuit devices.